Glass-lined instruments are used with heating media such as stream, water, and heat conductive fluids passing through jackets. In a prolonged use of such jackets, however, scales based on iron oxides such as Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3 generate due to corrosion, and adheres to the inside surface of the jacket. Predominant in the use at relatively low temperatures is Fe.sub.2 O.sub.3, whereas Fe.sub.3 O.sub.4 is more frequently observed at high temperatures. The scale reduces heat conductivity and not only leads to a considerable decrease in productivity but also increases the use of stream or cooling water. Thus, periodical descaling of the jacket inside is desired.
Known methods for descaling of the jacket inside of glass-lined instruments include physical methods using high-pressure water, and chemical methods by dissolution removal using organic acid-based detergents.
In the case of using chemical detergents such as hydrochloric acid for descaling of the jacket inside of glass-lined instruments, hydrogen generated by reaction of the acid with the metal transports through the inner lattice of the metal structure and reaches the boundary of the lining glass and the steel plate where it stays and gradually increases the pressure to finally attain a force large enough to break the glass. Thus, acid impact is feared. Accordingly, generally preferred is a
physical method such as high-pressure water cleaning (see Shinko Faudler Catalogue, No. 702, "Handling and Maintenance of Glass-Steel Instruments").
In the case of physical cleaning methods such as high pressure water cleaning, a removable area from the open end is limited such that drilling of cleaning holes in the instrument is required. Thus, its recovery work should be made with the consumption of time and money. In addition, complete descaling cannot be expected and is less effective as compared with chemical methods. Application of chemical methods is also under way, however, those methods under trial are applicable only to scales at their initial stages of formation since they use relatively mild detergents based on such as organic acids. Generally, no effective results are anticipated when applied to practical removal of iron oxide base scale deposits.
The use of powerful detergents based on hydrochloric acid or the like may cause acid impact. Repairing of the broken lining glass due to the acid impact is difficult and also brings about a considerably large loss of opportunity. As a result, though there is needed removal of the scale to improve thermal efficiency and to increase productivity, no countermeasures are taken in the practical use.
As a solution to the above problems, the present inventors have made extensive investigations on methods to easily remove iron oxide base scales generated inside jackets of glass-lined instruments without giving any damage to glass linings, and found that a hydrochloric acid solution containing specific compounds is effective enough to remove the scales free from any acid impact.